Level: unify the 'does this level carry meaningful args?' predicate between __str__ and as_dict()

Follow-up to #963, which was deliberately scoped to the as_dict() round-trip.

Level now answers the same question two different ways:

• arc/level.py:200 (as_dict): any(v for v in self.args.values()) — retains args when at least one of the keyword/block slots holds content (this is what #963 fixed).
• arc/level.py:152 (__str__): all([val for val in self.args.values()]) — requires both slots to be populated.

Consequence: a keyword-only level (the common case, e.g. args={'keyword': {'opt': 'opt=(verytight)'}}) serialises its args correctly but stays silent about them in its string representation, so the args are invisible in logs and in anything that renders a level via str().

This is cosmetic — it affects log/repr text only, not the round-trip or job args — but the divergence is a trap for the next reader.

Suggested fix: extract one named helper (e.g. Level.has_args() -> bool) returning bool(self.args) and any(v for v in self.args.values()), and call it from both sites. A unit test asserting that a keyword-only level shows its keyword args in str(level) would pin the behaviour.

Start in arc/level.py at str (line 152) and as_dict() (line 200), then inspect the existing Level unit tests. Define one shared meaningful-args predicate and use it at both entry points. Done means a keyword-only level includes its keyword args in str(level) while as_dict() behavior remains covered.